XXX It probably makes sense to have a flag for bsd.kern.mk to avoid these
rules.
XXX IO_NDELAY seems to be the main reason for it, when used in a cdevsw
read or write "flag" context. Perhaps a redundant declaration
somewhere like sys/conf.h might help remove the need for vnode.h in
these device drivers in the first place.
This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
